Development of a liquid crystal based system which can provide optical modulation (e.g., image storage) under zero electrical or magnetic field conditions has been the subject of much research during the past twenty or so years. To qualify for use in this type of application, the liquids crystals must be capable of independent stable existence in at least two different molecular domain structural states under zero field conditions. To provide light modulation capability, the two different states must interact in a different manner with respect to incident electromagnetic radiation. Furthermore, the two states must be interconvertible via application of a suitable energy impulse if the system is to be employed as a medium for storage of variable data input on a continuing basis. The existence of the liquid crystals under zero field in two different conformations has been referred to as "bistability". An even more favorable situation would be one in which the liquid crystals could exist in a range of conformations under zero field conditions in which each conformation interacts to a different extent with incident electromagnetic radiation. This type of situation would be called multistability. Multistable behavior has the advantage that it would provide a range of light attenuation resulting in what is commonly called gray scale capability.
Starting with the early 1970's there have been a number of attempts to produce zero field bistable liquid crystal systems. Previous examples would include U.S. Pat. No. 3,703,331 to Goldmacker et al., issued Nov. 21, 1972, and U.S. Pat. No. 3,821,720 to Greubel et al., issued Jun. 28, 1974. Although both inventions describe liquid crystal systems which exhibit a level of bistability, the existence of this zero field bistability was only temporary. In fact, the more light scattering of the two states decayed slowly to a transmissive state when held under zero field conditions. Obviously a decay process of that type represents severe limitations in the use of such materials as a "storage" system. An additional drawback with these devices is that two distinctly different types of electrical signals are required to switch the liquid crystal structures back and forth between the two stable states.
U.S. Pat. No. 3,806,230 to Haas, issued Apr. 23, 1974, also describes a bistable liquid crystal system. In this patent a cholesteric liquid crystal system is shown to exist in either a planar transmissive state or in a single mesophase focal-conic light scattering texture under zero field conditions. However, the zero field stability of both states exhibit an acknowledged marked time dependence as shown by data revealed in this patent. For example, the liquid crystal texture identified as focal-conic shows an initial large increase in optical transmission during the first few minutes after removal of the electrical field with the transmission continuing to increase slowly with time thereafter. In fact, the focal-conic state is reported to have completely disappeared after approximately six hours under zero field conditions, again precluding use of such a system for long-term "storage" purposes. An additional practical limitation in this device is the relatively small difference in transmission reported between the two bistable states and this difference clearly decreases continuously under zero field conditions. In practical terms, this small difference in transmission corresponds to a poor initial contrast ratio which becomes progressively worse with time under zero field conditions. Finally, it is important to note that no gray scale capability is available with these systems as no intermediate zero field stabilities are reported (i.e., the system employed is not multistable).
A class of liquid crystal materials well known to exhibit bistability are those compounds known as ferroelectric liquid crystals (FLC). Examples of recent patents in which FLC devices are proposed for use to record information as a stored liquid crystal image, but not under zero field conditions, are U.S. Pat. No. 5,046,830 to Nakanowatari, issued Sep. 10, 1991, and U.S. Pat. No. 5,272,552 to Yoshimaga et al., issued Dec. 21, 1993. In common with all other FLC systems, these displays are limited to only two different field states (i.e., they are strictly bistable in nature) at a given applied voltage. This bistability is not stable under zero field conditions. The inherent availability of only two states eliminates gray scale capability with these FLC displays. Additional disadvantages of these systems are the requirements of the use of a polarizer and backlighting to obtain readability and sufficient contrast ratio under normal operating conditions.
U.S. Pat. No. 4,333,708 to Boyd et al., issued Jun. 8, 1982, describes a mechanically multistable liquid crystal cell based on liquid crystals specifically in the nematic mesophase structures. This display requires the use of polarizers and also requires dual frequency address modes to establish the bistability. A serious disadvantage of this display is the strong temperature dependence of the frequency addressing step. Although the patent title refers to multistability, this title is misleading as a given display exhibits only bistable behavior. To achieve the multistable states, a series of different displays, each having a particular surface boundary condition, must be constructed. Each of these individual displays would then exhibit a particular equilibrium liquid crystal domain of local energy minima. This does not provide true zero field multistability as provided in the present invention.
Another recent patent involving a zero field stable liquid crystal system is U.S. Pat. No. 5,251,048 to Doane et al., issued Oct. 5, 1993. This patent references long-term zero field bistability using a chiral nematic liquid crystal system. This bistability is predicated specifically on the presence of a polymer additive to the liquid crystal mix to create "polydomains of polymer network dispersed throughout the cholesteric liquid crystals". Additional publications dealing with this topic are the papers of Yang and Doane (1992), Doane et al. (1992), Fung et al. (1993) and Yang et al. (1991). The presence of this polymer additive not only increases the display complexity but, in fact, adversely affects key display physical properties. For example, as noted explicitly by Yang and Doane (1992), the polymer network used to create the polydomains scatters light which significantly lowers the optical transmission ratio (i.e., contrast ratio) attainable between the bistable states. The presence of this polymer network also functions to lower the response time and reduce the viewing angle of these displays.
At present, commercial applications requiring retention of liquid crystal images utilize primarily supertwist nematic (STN) or active matrix thin film transistor (AMTFT) technologies. Both of these approaches involve major disadvantages and limitations. For example, these displays require use of polarizers and other light attenuating components thus creating high power consumption backlighting requirements. This is a severe disadvantage in many applications as in operation of portable notebook type displays. The AMTFT displays are not true zero field image storage systems as they require a constant power input for image refreshing. The STN displays do not possess inherent gray scale capability as a result of the extreme steepness of the optical voltage response curve of the liquid crystals employed. Although gray scale can be achieved, it is obtained at the expense of display resolution by using, for example, four pixels instead of only one per area. Anywhere from one to four pixels are activated at a particular time to provide the gray scale effect. The AMTFT devices use semiconductors to provide memory effects and involve use of expensive, ultra high resistance liquid crystal materials to minimize RC losses. Additionally, these displays are both difficult and costly to produce and they are, at present, limited to relatively small size displays. Both STN and AMTFT systems require the use of filters to achieve full color capability, thus creating further light attenuation and increased backlighting needs.